1. Field of the Invention
The present invention is related to a television signal transmitter for converting a television intermediate frequency signal into a television signal to be transmitted.
2. Description of the Related Art
FIG. 4 schematically shows an arrangement of a conventional television signal transmitter. An input signal of a television signal transmitter is entered via a band-pass filter 11 to a first mixer 12a. As to this input signal, a picture intermediate frequency is selected to be 45.75 MHz and a sound intermediate frequency is selected to be 41.25 MHz in the US television technical specification. The first mixer 12a constitutes a first frequency converter 12 in combination with a first local oscillator 12b. Then, a first local-oscillator signal having a frequency of 1065 MHz is entered from the first local oscillator 12b into the first mixer 12a. The first mixer 12a mixes the input signal with the first local-oscillator signal so as to produce an intermediate frequency signal base upon a summation of the frequencies of these signals (namely, picture intermediate frequency is 1110.75 MHz, and sound intermediate frequency is 1106.25 MHz).
The intermediate frequency signal is entered via a band-pass filter 13 to a second mixer 14a. The second mixer 14a constitutes a second frequency converter 14 in combination with a second local oscillator 14b. Then, a second local-oscillator signal outputted from the second local oscillator 14b is inputted to the second mixer 14a. The frequency of this second local-oscillator signal is changed from approximately 1160.75 MHz up to approximately 2110.75 MHz. As a result, in the second mixer 14a, the intermediate frequency signal is mixed with the second local-oscillator signal, so that such a television signal is outputted in any one of channels covered within a predetermined output frequency band defined from 50 MHz up to 1000 MHz, corresponding to a difference between the frequency of the intermediate frequency signal and the frequency of the second local-oscillator signal.
This television signal is transferred via a band-pass filter 15 to a cable of a CATV system. A passband of this band-pass filter 15 is selected to be 50 MHz up to 1000 MHz.
The intermediate frequency signal outputted from the first mixer 12a is entered via the band-pass filter 13 to the second mixer 14a. Since the frequency of this intermediate frequency signal is high, harmonics of the intermediate frequency signal cannot be sufficiently attenuated. As a result, when the harmonics of the intermediate frequency signal are entered into the second mixer 14a, interference signals are generated from the second mixer 14a. Among these interference signals, double higher harmonics are most typically produced from the second mixer 14a. When the output frequency becomes nearly equal to a half of the frequency of the intermediate frequency signal, the interference signals are generated within the frequency band of this channel, which will be described in detail.
Assuming now that the picture intermediate frequency is defined as “FI”, and also the frequency of the second local-oscillator signal is defined as “FL”, an output frequency “FO” of a desirable television signal outputted from the second mixer 14a is expressed by the following formula (1):FO=FL−FI  (1).
Also, a frequency “FU”, of an interference signal outputted from the second mixer 14a is expressed by the below-mentioned formula (2):FU=2FI−FL  (2).
In this case, since interference may occur when the output frequency “FO” of the television signal in the formula (1) becomes equal to the frequency “FU” of the interference signal in the formula (2), if this condition is acquired, then the following formula (3) is given:FU=FI−FO  (3).
That is to say, the frequency “FU” of the interference signal may be defined as such a frequency obtained by subtracting the output frequency “FO” from the picture interference frequency “FI.” As a consequence, when the output frequency “FO” becomes near equal to ½ of the picture intermediate frequency “FI”, the interference signal may be produced within the band of this channel. Since the frequency “FU” of this interference signal is generated within the output frequency band, this interference signal cannot be removed by the band-pass filter 15 which is provided on the output side of the second mixer 14a. 
For instance, when the picture intermediate frequency “FI” is equal to 1110.75 MHz, the interference signal having the frequency “FU” is generated at such a frequency nearly equal to ½ of this picture intermediate frequency “FI”, namely approximately 555 MHz. In an actual case, as indicated in FIG. 3, since the picture carrier frequency of the TV channel “CH.79” is located at the frequency of 553.25 MHz in the US television technical specification, the interference signal having the frequency “FU” is produced at a frequency of 557.5 MHz calculated from the above-described formula (3). This frequency of the interference signal corresponds to an upper-sided frequency of the picture carrier frequency of the channel CH.79, and also corresponds to such a frequency within the frequency band of this TV channel. As a consequence, when the TV signal of this channel “CH.79” is received, adverse influences caused by the interference signal having the frequency “FU” may appear.
On the other hand, since the frequency “FU” of the interference signal becomes nearly equal to ½ of the picture intermediate frequency “FI”, the frequency of the intermediate frequency signal may be increased to be set to two times, or more higher than the frequency of the television signal of the maximum output channel (namely, highest frequency of output frequency band). However, increasing the frequency of such an intermediate frequency signal could not provide the best solution as to the interference signal in view of circuit loss and the like.